This project exemplifies our continuing efforts to delineate the mechanism by which the avian retrovirus RNA-directed DNA polymerase converts the RNA genome into covalently closed circular proviral DNA. Our proposed studies specifically address the involvement of the location of the primer RNA, RNase H activity, and terminally repeated genomic nucleotide sequences in the synthesis and circularization of proviral DNA. To this end we plan to continue to rigorously test our original model of proviral DNA synthesis as a viable means by which the reverse transcriptase converts the viral genome into covalently-closed circular DNA. Modifications of this model are presented which are compatible with both the diploid nature of the viral genome and the structure of intermediate forms of proviral DNA identified in virus-infected cells. The experimental approach employed in these studies involves analysis of the structure of DNA transcripts and their mechanism of synthesis in reconstructed reactions containing purified viral RNA and reverse transcriptase in vitro. We plan to continue to elucidate (1) the role of the genomic terminally redundant nucleotide sequences in the synthesis of genome-length DNA; (2) the role of RNase H in the continued synthesis of DNA at the 3' end of the viral genome and the synthesis of "plus" strand DNA; (3) the mechanism of "plus" strand DNA synthesis; and (4) the mechanism of formation of covalently closed circles. Methodology employed in these studies includes RNase Tl oligonucleotide mapping, nucleotide sequence analysis, molecular hybridization, electron microscopy, restriction endonuclease mapping, polyacrylamide gel electrophoresis and various physicochemical and enzymological methods presently available for the analysis of DNA.